The present invention relates to a method and an apparatus for measuring the twisting state of an optical fiber in a method for generating twisting in the optical fiber during the manufacturing of the optical fiber in order to obtain an optical fiber having small polarization mode dispersion.
This application is based on Japanese Patent Applications (Patent Applications. Nos. Hei 11 (1999)-135043 and 2000-128307), and the contents of these Japanese Patent Applications are incorporated as parts of this specification.
Heretofore, with regard to the manufacturing of an optical fiber by drawing a cylindrical preform made of silica based glass, a method for obtaining an optical fiber having small polarization mode dispersion (PMD) by generating twisting in the optical fiber has been proposed.
For example, a method for carrying out drawing while rotating a preform at a high speed is proposed in Japanese Patent Laid-Open No. Hei 8 (1996)-59278.
In Japanese Patent Laid-open No. Hei 8 (1996)-295528, disclosed is a method for generating twisting in a primary coated optical fiber by guiding an optical fiber having a primary coating layer formed thereon with a swing guide roller reciprocated to change the inclination of a rotary shaft, and then guiding the same with the guide roller having the rotary shaft fixed thereto.
In Japanese Patent Laid-Open No. Hei 9 (1997)-2834, disclosed is a method for generating twisting in a primary coated optical fiber by guiding an optical fiber having a primary coating layer formed thereon by using a roller adapted such that an angle between a rotary shaft direction and a circumferential surface can be changed in a circumferential direction.
In Japanese Patent Indication No. Hei 10 (1998)-507438, proposed is a method for adding twisting to an optical fiber by moving two pulleys forward and backward in opposing directions along a direction perpendicular to the longitudinal direction of the optical fiber in a state where the optical fiber having a coating layer formed thereon is held between the two pulleys rotated in opposing directions.
Incidentally, the PMD characteristic of an optical fiber manufactured while adding twisting as described above is decided by the twisting state of an optical fiber, for example, in which direction twisting is generated and how many times the twisting is generated in the optical fiber of a fixed length. However, no proposals have been made regarding a method for measuring the state of twisting added to an optical fiber. Therefore, the twisting state of an optical fiber has heretofore had to be predicted from the apparent number of rotations calculated based on manufacturing conditions including the number of rotations of a preform, the moving speed of a pulley, and the like. However, there has not always been coincidence between the apparent number of rotations and the actual number of rotations (substantial number of rotations), and thus the conventional method has lacked accuracy.
The present invention was made considering the foregoing problems, and it is an object of the invention to enable the actual twisting state of an optical fiber manufactured while adding twisting to be measured accurately.
A method for measuring twisting of an optical fiber of the present invention has a feature in that an outer diameter of the optical fiber is continuously measured along the longitudinal direction of the optical fiber from two different directions in a plane perpendicular to the longitudinal direction of the optical fiber.
An apparatus for measuring twisting of an optical fiber of the present invention comprises: means for continuously measuring an outer diameter of the optical fiber along the longitudinal direction of the optical fiber from two different directions in a plane perpendicular to the longitudinal direction of the optical fiber.
A method for manufacturing an optical fiber of the present invention comprises the steps of: forming the optical fiber by drawing a preform; generating twisting in the optical fiber; and continuously measuring an outer diameter of the optical fiber from two different directions in a plane perpendicular to the advancing direction of the optical fiber.
An apparatus for manufacturing an optical fiber of the present invention comprises: means for forming the optical fiber by drawing a preform; means for generating twisting in the optical fiber; and means for continuously measuring an outer diameter of the optical fiber from two different directions in a plane perpendicular to the advancing direction of the optical fiber.